The bed temperature profile of stirred, fixed-bed gasifier is conventionally obtained using a plurality of ungrounded thermocouples mounted in the arms of the stirrer. The stirrer moves up and down through the bed in a helical manner through all zones or layers, from an ash layer at the bottom, a fire zone, a devolatilization zone, and a raw coal layer at the top. As is explained in more detail hereinbelow, the lower part or layer of the solids bed consists of ash (reacted coal) and this layer must be maintained at a sufficient height above the grate of the gasifier to prevent grate burnout by the high reaction temperatures just above the ash layer. The thickness of the ash layer is controlled by rotation of the grate and the thermocouples referred to above are used to determine the temperatures within the bed, it being understood that the temperatures of the ash layer are much lower than that of the bed reactor layer (fire zone) immediately thereabove. By knowing the vertical positions of the stirrer arms and the temperature outputs of the thermocouples carried thereby, the thickness of the ash layer can be determined.
The corrosive hot environment within the bed occasionally causes a thermocouple to fail, e.g., by shorting to ground. This type of failure produces an overvoltage in the amplifier circuit which, in turn, drives the data link out of synchronization. In a typical system, a multiplexed FM/FM telemetry (wireless) data link is used and because of the loss of synchronism all bed temperature data is rendered useless due to the resultant garbling of the data.
A further problem with, or demand made on, such systems is that the thermocouple amplifier must be operated by a battery over long continuous time periods. Commercially available isolated amplifiers, such as the Burr-Brown Iso100AP isolation amplifier and 0722 DC/DC converter require 40 mA for each amplifier and for six amplifiers (corresponding to the number of thermocouples typically used), the total is 240 mA. This obviously would provide very high loading of the (typically 24-volt) battery employed in these systems particularly considering that the battery is required to operate continuously for long periods, e.g., 6 weeks.